It's slightly easier because native code is typically faster but I would not say that it would cause it to be the case where you can only do the attack by handwriting assembly. Rowhammer involves hitting memory locations which is generally easy to do from any context (most languages have arrays…). It's not like you need to do weird branch prediction stuff which might require specific gadgets to be present.

Fine-grained machine code is likely to build more efficient/successful exploit.

But the goal I guess is to run that in one of the whatng cartel web engines (aka "sneaky" javascript), which are by themselves a security flaw already...